This invention relates generally to radio frequency (RF) excited gas plasma reaction devices.
Low temperature ionized gas, or cold plasma, has become widely employed for reaction with non-gaseous substances for processing as well as analytic purposes. In a low-pressure oxygen plasma, for example, substances can burn or oxidize at near room temperatures that otherwise would require 10 times the temperature for the same reaction. Apparatus called reactors are generally used for plasma generation. In a continuously pumped chamber or reacter, oxygen at low pressures, e.g. 1 mmHg, is energized by RF power.
Since about 1976, interest in plasma reactors has been stimulated by Integrated Circuit (IC) fabrication technology. It became apparent that manufacturing steps such as photo-resist stripping and wafer material removal, or etching, could be accomplished in a plasma discharge with significant advantages. As commercial plasma machines became available to IC manufacturers, work began to appear on the use of plasma in failure analysis such as in ICP Operating Bulletin #73, Stripping Wafers with Plasma (1978), International Plasma Corporation, Hayward Calif.; Plasma Etching, Circuits Manufacturing April 1978, p. 39, Thickness Variance of Spun on Photo Resist, Circuits Manufacturing, April 1978, p. 71; and Decapsulation of Epoxy Devices Using Oxygen Plasma, D. D. Wilson et al, Annual Reliability Physics Syniposium Proceedings, 1977, p. 82. While plasma is quite competitive in removing certain organics such as photo resists, it is less so with respect to stripping plastic encapsulants. This is partly, but not entirely, due to the material thickness involved.
At a particular point in the failure analysis of an encapsulated IC, the IC must be opened for the analysis to continue. This presents the problem of removing the encapsulant as completely as possible with a minimum or no alteration to the underlying circuit. For hybrid IC's the removal problem is aggravated by the nature and makeup of a hybrid which is a complex, multicomponent microcircuit comprised of many materials. Chemical strippants have been successful with some plastic devices but are not usable on hybrid IC encapsulants because of the circuit materials and resultant component damage.
Low temperature plasma stripping can be used to strip or ash encapsulants utilized in hybrid IC's. Reaction temperatures appear to be compatible with hybrid components and no serious material problems are apparent. However, the process is extremely slow, taking as long as three or more work days to ash the encapsulants.
The usual accelerating agent CF.sub.4 cannot be used since it has been found to damage the IC. Additionally, the relatively thick encapsulants cannot easily be mechanically or otherwise thinned prior to plasma exposure.
Commercial plasma reactors are generally available in two basic configurations: one having a planar geometry with parallel internal electrodes and the other being a barrel reactor having cylindrical and external electrodes. The behaviour and performance of each type is very different.
The equipment to produce and contain a plasma involves three fundamental functions: a reaction chamber exhausted via a vacuum system, provision for a regulated gas flow, and excitation. The reaction chamber is electrically discharged by means of Radio Frequency (RF) energy. The gas is maintained at low pressure by continuous pumping and is excited through electron-molecular collisions driven by the RF energy; that is, electrons and ions respond to the RF electronic field, and their collisions bring about various excited molecular and atomic species. It is through these dissociated and excited species that highly activated chemistry may take place.
Gas discharges, often called glow discharges, have been studied for more than 100 years. Early work used DC discharges; RF and microwave discharges are more common today. Plasma discharges, either DC or RF, involve highly complex phenomena; these phenomena are generally understood now. Of major complexity and importance are the electron, ion, and atomic distributions in a discharge. These are a function of electronic field distribution, gas flow, pressure, materials in contact with the plasma (including specimens), geometrics of the apparatus, applied power, and others.
J. L. Vossen in Fourth International Plasma Chemistry Conference Proceedings, pp. 344, 1979, International Union of Pure and Applied Chemistry ISPC-IV; and Jour. Elec. Chem. Sol., 126 No. 2, February 1979, p. 319, discussed glow discharge phenomena relative to plasma conditions, emphasizing that a glow discharge is not simply a source of electrons that dissociates molecules. In a review publication, Vossen describes the merits of the Reinberg radial gas flow system described in U.S. Pat. No. 3,757,733, which accommodates the co-creation and consumption of reactive species. Flow and flow rate considerations are most important for reaction uniformity. Vossen points out that gas flow is not a simple, independent parameter; other parameters are equally complex. Vossen discusses at some length an important phenomenon found in RF discharges. A direct current bias potential is produced at internal electrodes as a result of the inability of the massive ions to follow the RF field with the same mobility as the electrons. In a barrel reactor, bias potentials do not form since electrodes are not in direct contact with the plasma. Common to both reactor types, however, are ion or dark space sheaths that result at walls and at internal electrodes. These sheaths are important and play a large role in active species generation and recombination. The sheaths are pressure dependent, and changes in pressure will thereby strongly affect active species. Specimen surfaces also interact with the plasma, adding to the difficulty of anticipating reactions and rates. Sheaths can also form on specimens.
Examples of various prior art reactors and methods of ashing various substances are taught in U.S. Pat. Nos. 3,795,557; 4,017,404; 4,134,817; 4,073,669; 3,875,068; 3,705,091; 4,066,037, 3,647,676, 3,879,597; 3,616,461; 3,619,403; 3,806,365; 4,028,155; and 3,671,195.